Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling, diurnal emissions, and running loss vapors and then purge the stored vapors during a subsequent engine operation. Specifically, the fuel vapors are stored in a fuel vapor storage canister packed with an adsorbent (e.g., activated carbon) that adsorbs and stores the vapors until they are routed to an engine intake manifold for use as fuel. The fuel vapors may be comprised of vaporized hydrocarbons having a range of carbon chain lengths. The ability of the fuel vapor storage canister to adsorb fuel vapors is enhanced at cooler temperatures, and the ability of the fuel vapor storage canister to desorb fuel vapors is enhanced at hotter temperatures. Lower molecular weight hydrocarbons (also referred to herein as “light ends”), such as one-carbon methane to four-carbon butane, may easily desorb during purging as fresh air is flowed through the fuel vapor storage canister. However, higher molecular weight hydrocarbons (also referred to herein as “heavy ends”), such as seven-carbon heptane and above, resist movement during purging without heat to energize them to desorb from the adsorbent. As a result, fuel vapor storage canister cleaning may be incomplete during purging due to trapped heavy ends if heat is not applied. Incomplete fuel vapor storage canister cleaning may lead to greater bleed emissions. For example, when the vehicle soaks in the sun, the fuel vapor storage canister may heat up enough for the heavy ends to desorb and escape to the atmosphere.
Furthermore, in stop/start vehicles in which the engine is shutdown while the vehicle remains on, such as when the vehicle is stopped, frequent engine shutdowns may reduce opportunities to purge the fuel vapor storage canister. Reduced opportunities for purging may lead to a high vapor load on the fuel vapor storage canister, increasing bleed emissions. Therefore, it may be beneficial to quickly purge both light and heavy ends as soon as possible to ensure that the fuel vapor storage canister is effectively cleaned.
Other attempts to reduce bleed emissions by effectively purging hydrocarbon heavy ends include using heated fuel vapor storage canisters. One example approach is shown by Peters et al. in U.S. 20150090232 A1. Therein, a method is disclosed for adjusting a heater of a fuel vapor storage canister based on a rate of purge flow exiting the fuel vapor storage canister and a concentration of hydrocarbons released from the fuel vapor storage canister. Furthermore, to counteract relative low temperatures reached by the fuel vapor storage canister during engine-off periods (e.g., when the engine is not rotating and not combusting air and fuel) in a hybrid vehicle, where heating the fuel vapor storage canister during purge may result in ineffective purging, the heater of the fuel vapor storage canister may be maintained at a temperature that is lower than that desired for purge but higher than ambient temperature.
However, the inventor herein has recognized that maintaining the fuel vapor storage canister heater on, even at a low level, consumes energy. As one example, it is unknown when or if the engine will turn back on during the drive cycle for purging to occur. Therefore, the fuel vapor storage canister may be heated unnecessarily, which may waste energy.
In one example, the issues described above may be addressed by an evaporative emissions method, comprising: while a vehicle remains off, preheating a fuel vapor storage canister in an evaporative emissions system of the vehicle to a desired temperature at a determined duration prior to commencing a drive cycle; and purging vapors stored in the fuel vapor storage canister to an intake of an engine of the vehicle after the commencement of the drive cycle. In this way, cleaning of the fuel vapor storage canister may be expedited.
As one example, the drive cycle may be a learned drive cycle that that enables the purging to occur within a threshold duration after the commencement of the drive cycle. For example, the purging may be enabled if engine is operated above idle speed and with vacuum in the intake above a threshold vacuum. Further, prior to the commencement of the drive cycle and while the fuel vapor storage canister is preheated, an evaporative emissions system test may be performed to determine if the evaporative emissions system is degraded or not. For example, if the pressure of the evaporative emissions system does not reach a threshold pressure, the evaporative emissions system may be determined to be degraded. By heating the fuel vapor storage canister only when conditions for purging the fuel vapor canister are anticipated to be met, energy may be conserved. Further, by performing the evaporative emissions system test while the fuel vapor storage canister is preheated, even if purging does not occur, the energy spent preheating the fuel vapor storage canister may be utilized.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.